iPhone 4S - iOS 5.1 - iMessage not working?

I updated to iOS 5.1 and used the txt message GiffGaff sent me for my internet settings - I can go online using 3G but when I go into Settings / Messages - and try swicth on iMessage just asks me to sign in, time and time again?

I'm using iMessage on my Messages App on the Mac, so iMessage does work fine just not on my iPhone after the update, any suggestions?